1. Field of the Invention
The present invention relates to a head position control method, a disk device and a servo track write method for positioning a head for recording/regenerating data on a disk on a target track of the disk, and more particularly to a head position control method, disk device and servo track write method for demodulating servo signals recorded on an eccentric disk and obtaining the position.
2. Description of the Related Art
The disk storage device which records or regenerates data to/from a rotating disk medium is widely used as a storage device for data. As FIG. 18 shows, the disk device is comprised of a disk 94 for storing data, a spindle motor 96 for rotating the disk 94, a head 90 for recording/regenerating information on the disk 94, and an actuator 92 for moving the head 90 to a target position. Typical of such a device is a magnetic disk (HDD: Hard Disk Drive) and an optical disk device (DVD-ROM, MO).
In the magnetic disk device, position signals 100 for detecting a position of the head 90 are recorded on the disk 94. The position signal 100 is comprised of a servo mark, track number and offset information. The current position of the head 90 can be obtained using the track number and offset information.
The difference between this position information and the target position is determined, and calculation is performed according to the position error amount, so as to supply the drive amount for driving the actuator 92, such as current in the case of a VCM (Voice Coil Motor), and voltage in the case of a piezo-electric actuator.
To record a servo signal (position signal) 100 on the disk 94, a method for recording the servo signal by an external STW (Servo Track Write) device has been proposed instead of a conventional STW method, where the disk device itself records the servo signal. For example, this method was proposed in the Japanese Patent Laid-Open No. 03-073406 “Servo information writing method for a magnetic disk device”, (published on Mar. 28, 1991).
If the disk 94 for which STW was performed externally is mounted on an HDD device, eccentricity occurs, as shown in FIG. 19, and the position of the head 90 following up the position signal 100 oscillates in a sine wave manner along with the rotation of the disk 94. In other words, it is extremely difficult to set the disk 94 on the spindle motor 96 with perfectly matching the rotation center 94-1 when the position signals 100 are written on a circumference on the disk 94 with the axial center 98 of the spindle motor 96. Therefore the shift is occurred between the rotation center 94-1 and the axial center 98.
If the head follows up to this shift, that is to this eccentricity, the head is constantly oscillating (drive current is flowing), which increases power consumption and tends to make the operation to switch the head unstable. To solve this problem, methods for controlling the actuator without following up the eccentricity have been proposed. For example, a method has been proposed in Japanese Patent Laid-Open No. 9-128915 (published on May 16, 1997), and in Japanese Patent Laid-Open No. 9-330571 (published on Dec. 22, 1997).
It is stated in such proposals that a position orbit (virtual circular orbit) is provided so as to ignore the eccentricity, as shown in FIG. 20, removing this orbit from the demodulation signals of the head, in order to obtain a demodulation position, and the actuator is controlled with the demodulation position. By this, as FIG. 19 shows, the head 90 is positioned on the circular orbit 110 with the rotation axis 98 of the spindle motor as the center, using the position signal with respect to the circular orbit of the eccentric position signal 100, and reads data from/writes data to the disk 94. The orbit 110 of the head 90 is represented with a line, as shown in FIG. 21, then the orbit 110 crosses the orbit 102 of the position signal indicated by a sine wave.
An area demodulation method, where 2-phase signals PosN and PosQ are used for the position signals, has been in use. FIG. 22 is a diagram depicting 2-phase servo position signals, FIG. 23 is a block diagram depicting the position demodulation circuit thereof, and FIG. 24 to FIG. 26 are diagrams depicting the position demodulation signals.
As FIG. 22 shows, the position signal (servo signal) is comprised of a servo mark, gray code (track number), index, and offset signals (PosA−PosD). As FIG. 23 shows, the track number and the offset signals (PosA−PosD) are separated from the position signals from the head 90 in the block 120, and the 2-phase servo signals PosN and PosQ are calculated as follows.PosN=PosA−PosBPosQ=PosC−PosDThe position demodulation is performed so that either PosN or PosQ, whichever is smaller in the block 122, is used as Pos1. In other words, as shown in FIG. 22, either PosN or PosQ, whichever is smaller, is selected.
This means that the amplitude of the read output of each offset signal (PosA−PosD) from the head 90 is in proportion to the area of the offset signal (PosA−PosD) at the position of the head 90. In other words, this servo signal can demodulate the position of the head by demodulating the area indicated by amplitude.
The position sensitivity gain 124 changes the gain according to the track position. Such a demodulation method is described in detail in Japanese Patent Laid-Open No. 8-195044 (published on Jul. 30, 1996), for example.
In the case of the 2-phase servo signals of this area demodulation method, a crossover (switching) of PosN and PosQ is generated during demodulation, as shown in FIG. 24. If the head 90 crosses the position signal diagonally, as shown in FIG. 22, that is when the head 90 has velocity, an error occurs to the demodulation positions by PosN and PosQ, and a velocity offset of NQ is generated. In other words, the 2-phase servo signals PosN and PosQ can be observed with a ¼ phase shift in the track direction, as shown in FIG. 24, only when the velocity of the actuator is zero.
For example, FIG. 25 shows the result of simulating the status of PosN and PosQ when the head moves at a 20 track/sample in the device. As FIG. 25 shows, the phase relationship between PosN and PosQ is shifted. FIG. 26 shows the result of calculating the demodulation position at this time, and the switching of PosN and PosQ makes correct position demodulation impossible.
As FIG. 25 and FIG. 26 show, offset is different between the demodulation blocks of PosN and PosQ. To correct offset due to the velocity, Japanese Patent Laid-Open No. 2001-256741 (published on Sep. 21, 2001), for example, discloses inputting the velocity V of the actuator into the block 122, in order to correct PosN and PosQ.
However, in the above mentioned velocity offset correction method, PosN and PosQ are corrected by the velocity V of the head obtained from the demodulation position in FIG. 20. On the other hand, as FIG. 19 and FIG. 21 show, eccentricity is ignored in the virtual circular control, so the velocity correction values for the position signals PosN and PosQ by eccentricity are different from those acquired from the demodulation positions.
Therefore in the case of virtual circular control, accurate velocity offset correction is difficult. Track pitch in particular is currently narrow because of the increase in recording densities, where the number of eccentric tracks increases and higher precision positioning is requested, so the difference in correction velocity can no longer be ignored.